The present invention relates to a control unit, a power supply device and an electronic apparatus, and in particular, to a power supply device having a converter that converts a voltage by switching, a control unit for controlling the power supply device, and an electronic apparatus.
A DC-DC converter is used as a power supply device for obtaining a desired DC voltage from a DC power supply. There is a natural commutation type converter as the DC-DC converter. FIG. 1 is a circuit diagram illustrating a related-art step-down and natural commutation type converter, and FIG. 2 is a timing chart of the converter. Referring to FIG. 1, an FET 12 that is a switching circuit is connected between an input node Nin, to which a power supply voltage VB is applied, and an intermediate node N1. A diode 13 and a capacitor C2 are connected in parallel between the intermediate node N1 and a ground. An inductor L1 is connected between the intermediate node N1 and an output node Nout, and an electrolytic capacitor C1 is connected between the output node Nout and the ground.
Referring to FIG. 2, until time T1, the FET 12 is in an ON state and an intermediate voltage Vx at the intermediate node N1 is approximately the power supply voltage VB. During this period, an inductor current IL flowing through the inductor L1 in the direction from the intermediate node N1 to the output node Nout increases. When the FET 12 is turned off at time T1, the intermediate voltage Vx changes to a turn-on voltage of the diode 13 in the forward direction thereof. The inductor current IL decreases and becomes 0 at time T2. After time T2, electric charges accumulated in the electrolytic capacitor C1 flow through the inductor L1 from the output node Nout to the intermediate node N1, such that the negative inductor current IL flows. As a result, electric charges are accumulated in the capacitor C2, and the intermediate voltage Vx increases. Thus, the inductor current IL and the intermediate voltage Vx resonate. At time T3, when the intermediate voltage Vx reaches the power supply voltage VB, the FET 12 is turned on. That is, a zero voltage switching is performed. As a result, the inductor current IL flows again in the direction from the intermediate node N1 to the output node Nout. Thus, since the FET 12 is repeatedly turned on and off, the intermediate voltage Vx repeats between the power supply voltage VB and the turn-on voltage of the diode 13. The output voltage Vout becomes a DC voltage by a smoothing circuit including the inductor L1 and the electrolytic capacitor C1. Thus, the DC power supply voltage VB is converted into the DC output voltage Vout. In addition, when the FET 12 is turned on, a difference of electric potentials at both ends of the FET 12 is 0 (zero voltage switching). Accordingly, a power loss occurring due to switching can be suppressed.
Patent Document 1 discloses that a dead time is applied at the time of ON/OFF switching in order to make a switching frequency constant. Patent Document 2 discloses a method of adjusting a dead time that is a time at which both elements are turned off at the time of a soft switching operation, in order to adjust a dead time period.
Patent Document 1: Japanese Patent Publication No. 7-46853A
Patent Document 2: Japanese Patent Publication No. 2001-258269A
For example, in the step-down converter shown in FIG. 1, the intermediate voltage Vx does not reach the power supply voltage VB in a resonating state between time T2 and time T3 (refer to FIG. 2) if the output voltage Vout is equal to or smaller than ½ of the power supply voltage VB. In this case, the difference of electric potentials between both ends of the FET 12 does not become 0, and accordingly, the zero voltage switching cannot be performed. Thus, if the zero voltage switching cannot be performed, the power loss increases.